Impression made by Napoleon in the field? (4-4)

'the field?' is the definition.
I don't know anything about this answer so I cannot judge whether this works.

'impression made by napoleon' is the wordplay.
'impression' becomes 'mark' (I have seen 'Visible impression ' mean 'mark' so perhaps 'impression' could also mean 'mark').
'made by' means one lot of letters go next to another (letters written or made by another set of letters).
'napoleon' becomes 'hoof' (I am not sure about this - if you are sure you should believe this answer much more).
'mark' put after 'hoof' is 'HOOF-MARK'.

'in' is the link.

Can you help me to learn more?